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Abstract:

The present invention relates generally to the detection of alcohol. The
present invention relates more particularly to the film bulk acoustic
wave resonator-based devices, and their use in the sensing of ethanol
and/or acetone. One aspect of the invention is a method for detecting
ethanol, acetone or both in a gaseous sample including: providing a film
bulk acoustic wave resonator having a zinc oxide piezoelectric layer;
exposing the film bulk acoustic wave resonator to the gaseous sample;
determining the resonant frequency of the film bulk acoustic wave
resonator; and determining the concentration of ethanol, the
concentration of acetone, or both in the gaseous sample using the
resonant frequency of the film bulk acoustic wave resonator.

Claims:

1. A method for detecting ethanol, acetone, or both in a gaseous sample,
the method comprising providing a film bulk acoustic wave resonator
having a zinc oxide piezoelectric layer; exposing the film bulk acoustic
wave resonator to the gaseous sample; determining the resonant frequency
of the film bulk acoustic wave resonator; and determining the
concentration of ethanol, acetone, or both in the gaseous sample using
the resonant frequency of the film bulk acoustic wave resonator.

2. The method according to claim 1, wherein the film bulk acoustic wave
resonator comprises: a first electrode layer; the zinc oxide
piezoelectric layer disposed on the first electrode; a second electrode
layer disposed on the zinc oxide piezoelectric layer; and a resonant
frequency measuring circuit operatively coupled to the first electrode
and the second electrode.

3. The method according to claim 2, wherein the zinc oxide piezoelectric
layer has a thickness in the range of 0.2 μm to 5.0 μm; the first
electrode layer has a thickness in the range of 0.1 μm to 1.0 μm;
and the second electrode layer has a thickness in the range of 0.1 μm
to 1.0 μm.

4. The method according to claim 2, further comprising a diaphragm layer
having a thickness in the range of 0.1 μm to 2 μm, upon which the
first electrode layer is disposed.

5. The method according to claim 1, wherein the film bulk acoustic wave
resonator comprises: a diaphragm layer suspended above a void space, the
diaphragm layer having a first side and a second side; a zinc oxide
piezoelectric layer disposed on the first side of the diaphragm layer; a
first electrode layer disposed on the zinc oxide piezoelectric layer; a
second electrode layer disposed on the second side of the diaphragm
layer; and a resonant frequency measuring circuit operatively coupled to
the first electrode layer and the second electrode layer.

6. The method according to claim 4, wherein the diaphragm layer has a
thickness up to 2.0 μm; the zinc oxide piezoelectric layer has a
thickness in the range of 0.2 μm to 5.0 μm; the first electrode
layer has a thickness in the range of 0.1 μm to 1.0 μm; and the
second electrode layer has a thickness in the range of 0.1 μm to 1.0
μm.

7. The method according to claim 2, wherein the zinc oxide piezoelectric
layer has a surface area in the range of 0.0025 mm2 to 0.2 mm.sup.2.

8. The method according to claim 1, wherein the zinc oxide layer is
substantially crystalline, with its wurtzite C axis is substantially
perpendicular to its opposed surfaces.

9. The method according to claim 1, wherein the gaseous sample contains
ethanol and acetone.

10. The method according to claim 1, further comprising providing a
resistivity-based ethanol and/or acetone sensor; exposing the
resistivity-based ethanol and/or acetone sensor to the gaseous sample at
substantially the same time as the film bulk acoustic wave resonator is
exposed to the gaseous sample; determining the resistivity of the
resistivity-based ethanol and/or acetone sensor; and using the
resistivity of the resistivity-based ethanol and/or acetone sensor along
with the resonant frequency of the film bulk acoustic wave sensor in
determining the concentration of ethanol, acetone, or both.

11. The method according to claim 10, wherein the use of both the
resistivity of the resistivity-based ethanol and/or acetone sensor and
the resonant frequency of the film bulk acoustic wave sensor in
determining the concentration of ethanol substantially cancels out any
effect of acetone on the determination of the concentration of ethanol.

12. The method according to claim 10, wherein the use of both the
resistivity of the resistivity-based ethanol and/or acetone sensor and
the resonant frequency of the film bulk acoustic wave sensor in
determining the concentration of acetone substantially cancels out any
effect of ethanol on the determination of the concentration of acetone.

13. The method according to claim 1, wherein the film bulk acoustic wave
resonator has a resonant frequency in the range of 0.2 GHz to 10 GHz.

14. The method according to claim 1, wherein the film bulk acoustic wave
resonator is configured to change its resonant frequency substantially
linearly with ethanol concentration throughout the range of about 100 ppm
to about 250 ppm.

15. The method according to claim 1, wherein the gaseous sample is the
breath of a human subject.

16. A breath alcohol analyzer comprising a film bulk acoustic wave
resonator having a zinc oxide piezoelectric layer; and a circuit adapted
to determine an ethanol concentration of a gaseous sample using a
resonant frequency measured by the resonant frequency measuring circuit.

17. A breath acetone analyzer comprising a film bulk acoustic wave
resonator having a zinc oxide piezoelectric layer; and a circuit adapted
to determine an acetone concentration of a gaseous sample using a
resonant frequency measured by the resonant frequency measuring circuit.

18. The analyzer according to claim 16, wherein the film bulk acoustic
wave resonator comprises: a first electrode layer; the zinc oxide
piezoelectric layer disposed on the first electrode; a second electrode
layer disposed on the zinc oxide piezoelectric layer; and a resonant
frequency measuring circuit operatively coupled to the first electrode
layer and the second electrode layer.

19. The analyzer according to claim 18, wherein the zinc oxide
piezoelectric layer has a thickness in the range of 0.2 μm to 5.0
μm; the first electrode layer has a thickness in the range of 0.1
μm to 1.0 μm; and the second electrode layer has a thickness in the
range of 0.1 μm to 1.0 μm.

20. The analyzer according to claim 18, further comprising a diaphragm
layer having a thickness up to 2 μm, upon which the first electrode
layer is disposed.

21. The analyzer according to claim 18, wherein the zinc oxide
piezoelectric layer has a surface area in the range of 0.0025 mm2 to
0.2 mm.sup.2.

22. The analyzer according to claim 17, wherein the zinc oxide layer is
substantially crystalline, with its wurtzite C axis is substantially
perpendicular to its opposed surfaces.

23. The analyzer according to claim 17, wherein the resonant frequency
measuring circuit is operatively coupled to a system for determining a
concentration of ethanol in a gaseous sample.

Description:

CROSS-REFERENCE TO RELATED APPLICATIONS

[0001] This application claims the priority of U.S. Provisional Patent
Application Ser. No. 61/296,696, filed Jan. 20, 2010, which is hereby
incorporated herein by reference in its entirety.

BACKGROUND OF THE INVENTION

[0002] 1. Field of the Invention

[0003] The present invention relates generally to the detection of ethanol
and/or acetone. The present invention relates more particularly to the
film bulk acoustic wave resonator-based devices, and their use in the
sensing of ethanol and/or acetone.

[0004] 2. Technical Background

[0005] Driving under the influence of alcohol is a serious traffic
violation; such behavior causes many accidents and deaths on the road.
Electrochemical breath alcohol analyzers are generally used as a quick
and reliable screening device at sobriety checkpoints and after motorists
are pulled over on suspicion of DUI. However, acetone can strongly
interfere with electrochemical detection. Acetone is generally considered
to be the only endogenous volatile organic compound that is a potentially
interfering substance in breath alcohol analysis. It is present in the
breath of a normal person, and in increased concentrations as the result
of prolonged fasting, use of ketogenic diets, or diabetes. Moreover,
breath acetone itself can be an analyte of interest for medical
diagnostic purposes. The analysis of exhaled breath for acetone can help
to provide an express non-invasive diagnosis of ketosis.

[0006] In conventional sensors, ethanol and acetone can interfere with one
another. Accordingly, drunkenness can wrongly be interpreted as ketosis,
and vice versa. Ethanol and acetone can be distinguished using
electrochemical or infrared instruments in commercial breath alcohol
analyzers. However, they are complex and expensive, and specific training
is required in order to become a proficient user. Resistivity-based metal
oxide sensors have been developed, which can have relatively simple
structures and can be cost effective and easy to use. Their main drawback
is that they can not effectively distinguish ethanol and acetone. Because
they use the change in resistivity as the gas detecting signal, both
ethanol and acetone will share a similar response: a decrease in
resistivity. Resistivity-based ethanol sensors based on zinc oxide thin
films have been extensively investigated. Special attention is given to
discriminating between ethanol and acetone due to their similar chemical
nature. However, as both gases can reduce the resistivity of the sensor,
the selectivity was not as high as desirable. Selected ion flow tube mass
spectrometry has shown great potential in real-time concentration
monitorying of acetone and ethanol in human breath. While it is highly
selective and sensitive, its high cost and limited portability hinder its
usefulness as a standard diagnostic tool.

SUMMARY OF THE INVENTION

[0007] In certain aspects, the present invention relates to ethanol and/or
acetone sensing using a zinc oxide-based film bulk acoustic-wave
resonator. Film bulk acoustic-wave resonators more generally have been
drawing considerable attention both as filters and as high sensitivity
mass sensors in recent years. In certain aspects of this invention, film
bulk acoustic-wave resonator is used to measure ethanol concentration in
the environment. This method can address one of the most challenging
problems in ethanol and/or acetone sensing: the discrimination between
ethanol and acetone. In certain aspects of the invention as described
herein, the resonant frequency of a zinc oxide-based film bulk
acoustic-wave resonator device increases with increasing acetone
concentration, but decreases with increasing ethanol concentration. This
opposite responsivity can allow ethanol and acetone to be distinguished
from one another.

[0008] According to one aspect of the invention, a method for detecting
ethanol and/or acetone in a gaseous sample includes: [0009] providing a
film bulk acoustic wave resonator having a zinc oxide piezoelectric
layer; [0010] exposing the film bulk acoustic wave resonator to the
gaseous sample; [0011] determining the resonant frequency of the film
bulk acoustic wave resonator; and [0012] determining the concentration of
ethanol, acetone or both in the gaseous sample using the resonant
frequency of the film bulk acoustic wave resonator.

[0013] According to another aspect of the invention, a breath alcohol
and/or acetone analyzer comprises a film bulk acoustic wave resonator
having a zinc oxide piezoelectric layer; and a circuit adapted to
determine an ethanol concentration, an acetone concentration, or both of
a gaseous sample using a resonant frequency measured by the resonant
frequency measuring circuit.

BRIEF DESCRIPTION OF THE DRAWINGS

[0014] The accompanying drawings are not necessarily to scale, and sizes
of various elements can be distorted for clarity.

[0015]FIG. 1 is a pair of schematic diagrams illustrating a proposed
sensing mechanism for (a) ethanol and (b) acetone;

[0016]FIG. 2 is a schematic cross-sectional view of a film bulk acoustic
wave resonator suitable for use according to certain embodiments of the
invention;

[0017]FIG. 3 is a schematic cross-sectional view of a film bulk acoustic
wave resonator suitable for use according to other embodiments of the
invention;

[0018]FIG. 4 is a schematic cross-sectional view of a film bulk acoustic
wave resonator used in the experiments described in the Examples;

[0019]FIG. 5 provides top and bottom schematic perspective views of the
film bulk acoustic wave resonator used in the experiments described in
the Examples;

[0020]FIG. 6 is a photomicrograph of the film bulk acoustic wave
resonator used in the experiments described in the Examples;

[0021]FIG. 7 is an x-ray diffraction pattern of the zinc oxide layer of
the device of FIGS. 4-6;

[0022]FIG. 8 is a graph showing the response of a film bulk acoustic wave
resonator to ethanol;

[0023]FIG. 9 is a graph showing the response of a film bulk acoustic wave
resonator to acetone;

[0024]FIG. 10 are graphs showing (a) concentrations of ethanol/acetone
mixtures; and (b) response to mixtures of ethanol and acetone;

[0025] FIG. 11 is a graph showing response to ethanol with and without
ultraviolet radiation;

[0026]FIG. 12 is a graph showing response to acetone with and without
ultraviolet radiation;

[0027]FIG. 13 is a schematic cross-sectional view of a film bulk acoustic
wave resonator suitable for use according to certain embodiments of the
invention; and

[0028]FIG. 14 is a schematic depiction of a process for making the film
bulk acoustic wave resonator of FIG. 13, and a photomicrograph of the
resonator so made.

DETAILED DESCRIPTION OF THE INVENTION

[0029] Zinc oxide is a promising material for ethanol sensing
applications. Ethanol sensors based on zinc oxide thin films have been
extensively investigated. See, e.g., P. P. Sahay et al., J. Mater. Sci.,
40 (2005), pp. 4791-4793, which is hereby incorporated herein by
reference in its entirety. Special attention has been paid to
discrimination between acetone and ethanol due to their similar chemical
nature and presence in the breath of subjects of breath alcohol tests.
For example, Kim and his coworkers used combinational solution deposition
to prepare various SnO2--ZnO thin film sensors, which exhibited
different sensitivities for acetone and ethanol. Sens. Actuators. B., 123
(2007), pp. 318-324, which is hereby incorporated herein by reference in
its entirety. However, as both gases reduced the resistivity of the
sensor, the selectivity was not as high as desirable. In this disclosure,
a zinc oxide-based film bulk acoustic wave resonator device is provided
which has opposing responses to acetone and ethanol. Such opposite
responses can advantageously provide for relatively higher sensitivity
and discrimination between acetone and ethanol.

[0030] Without intending to be bound by theory, the inventors propose the
following mechanism for the opposite response to ethanol and acetone.
Ethanol reacts with adsorbed oxygen ions on the ZnO surface and generates
water which is adsorbed by the ZnO, as shown in FIG. 1(a). Accordingly,
the density of the film increases, resulting in a frequency decrease. See
X. Qiu et al., Appl. Phys. Lett., 94 (2009), 151917, which is hereby
incorporated herein by reference in its entirety. Acetone, on the other
hand, reacts with the surface adsorbed oxygen ions on the zinc oxide film
and releases CO2 as a reaction product, as shown in FIG. 1(b). P. P.
Sahay, J. Mater. Sci., 40 (2005), pp. 4383-4385, which is hereby
incorporated herein by reference in its entirety. Accordingly, the
density of the film decreases, resulting in a frequency increase.

[0031] One aspect of the invention is a method for detecting ethanol
and/or acetone in a gaseous sample. The detection can be, for example, a
simple yes-no detection of a threshold level of ethanol, acetone or both,
or alternatively a determination of a concentration of ethanol, acetone,
or both. The concentrations of ethanol, acetone or both can themselves be
reported, or alternatively one or both of the concentrations can be
related to some other desired quantity (e.g., a blood alcohol level of a
subject, or ketosis level of a subject).

[0032] According to this aspect of the invention, one step in a method for
detecting ethanol, acetone, or both in a gaseous sample is the provision
of a film bulk acoustic-wave resonator having a zinc oxide piezoelectric
layer. Particular embodiments of such film bulk acoustic-wave resonators
are described in more detail below. The film bulk acoustic wave resonator
is exposed to the gaseous sample. The resonant frequency of the film bulk
acoustic wave sensor is determined. For example, in certain embodiments,
the resonant frequency of the film bulk acoustic wave sensor is
determined both before and after exposure to the gaseous sample, so that
the difference in resonant frequency upon exposure can be measured. In
other embodiments, the resonant frequency is determined only after
exposure to the gaseous sample; in such embodiments, a known value of the
initial resonant frequency can be used. In certain embodiments, the
resonant frequency is measured as a function of time, for example so that
the time scale of absorption/desorption of acetone and/or ethanol is
known.

[0033] Finally, the concentrations of ethanol, acetone, or both in the
gaseous sample are determined using the determined resonant frequency (or
frequencies) of the film bulk acoustic wave sensor. As described in more
detail below, and demonstrated by the examples herein, as a result of the
zinc oxide material used in the piezoelectric layer, the resonant
frequency of the film bulk acoustic-wave resonator increases with
exposure to acetone vapor, and decreases with exposure to ethanol vapor.
The person of skill in the art can use a calibration curve, for example,
to correlate the determined resonant frequency (or freqencies) with the
concentration of ethanol, the concentration or ethanol, or both. The
concentrations of ethanol, acetone, or both in the gaseous sample can be
determined as an actual concentration (e.g., in ppm). Alternatively, the
concentrations of ethanol, acetone, or both can be reported as to whether
they meet some threshold level (e.g., a level of ethanol that would
constitute driving while impaired). The concentration of ethanol can
alternatively or also be determined as some other value correlated with
the concentrations of ethanol, acetone, or both in the gaseous sample.
For example, a blood alcohol level can be determined from the
concentration of ethanol in the gaseous sample, or directly determined
from the determined resonant frequency (or frequencies). Similarly, a
concentration of acetone in a bodily fluid (e.g., in blood, or an
equivalent concentration to urine) can be determined from the
concentration of acetone in the gaseous sample, or directly determined
from the determined resonant frequency (or frequencies). As used herein,
the term "determining the concentration of ethanol in the gaseous sample"
includes the determination of any value or property correlated with
ethanol concentration in the gaseous sample, regardless of whether a
numerical value of ethanol concentration in the gaseous sample is
actually determined. Similarly, as used herein, the term "determining the
concentration of acetone in the gaseous sample" includes the
determination of any value or property correlated with acetone
concentration in the gaseous sample, regardless of whether a numerical
value of acetone concentration in the gaseous sample is actually
determined.

[0034] One embodiment of a film bulk acoustic wave resonator suitable for
use in the methods described herein is shown in schematic cross-sectional
view in FIG. 2. Film bulk acoustic wave resonator 200 includes a
diaphragm layer 205, suspended above a void space 207. The diaphragm
layer can, for example, be suspended by a substrate 206, as shown in the
embodiment of FIG. 2. A first electrode layer 210 is disposed on the
diaphragm layer 205, and a zinc oxide piezoelectric layer 220 is disposed
on the first electrode 210. A second electrode layer 215 is disposed on
the zinc oxide piezoelectric layer 220. A resonant frequency measuring
circuit 230 is operatively coupled to the first electrode 210 and the
second electrode 215. While in the embodiment of FIG. 2, the zinc oxide
piezoelectric layer is shown as being in contact with the electrodes, the
person of skill in the art will recognize that other layers (e.g.,
dielectric layers) can be disposed between the electrodes, between the
first electrode and the diaphragm layer, and/or on the second electrode.

[0040] Another embodiment of a film bulk acoustic wave resonator suitable
for use in the methods described herein is shown in schematic
cross-sectional view in FIG. 3. Film acoustic wave resonator 300 includes
a diaphragm layer 305, suspended above a void space 307, and having a
first side and a second side. A zinc oxide piezoelectric layer 320 is
disposed on the first side of the diaphragm layer, and a first electrode
layer 310 is disposed on the zinc oxide piezoelectric layer 320. A second
electrode layer 315 is disposed on the second side of the diaphragm
layer, such that the diaphragm layer and the zinc oxide piezoelectric
layer are both disposed between the first and second electrodes. A
resonant frequency measuring circuit 330 is operatively coupled to the
first electrode layer and the second electrode layer.

[0046] The diaphragm layer can be made of a variety of substances. For
example, in one embodiment, the diaphragm layer is made from silicon
nitride. In other embodiments, the diaphragm layer can be made from
silicon dioxide. The person of skill in the art can select other
substances for use in the diaphragm layer.

[0047] The devices described above with reference to FIGS. 2 and 3 include
a diaphragm layer. Similar devices can be made without a diaphragm layer.
In such embodiments, one or more of the other layers of the device can
extend to the substrate, thereby supporting the resonator structure. A
diaphragm layer can be used in the fabrication process to support device
layers as they are grown, then removed to provide a diaphragm-free
device.

[0048] The electrode layers can be made of a variety of substances. For
example, in certain embodiments, the second electrode layer is made from
gold (optionally deposited on a thin layer of chromium to enhance
adhesion). In certain embodiments, the first electrode layer is made from
aluminum. Of course, other materials can be used for the electrodes, such
as molybdenum, platinum, aluminum or gold/chromium.

[0049] The various elements can be formed in a variety of shapes and
sizes. As the person of skill in the art will recognize, the sensitivity
and resonant frequency of the device can depend on the shapes,
thicknesses and sizes of the various elements. For example, in one
embodiment, the zinc oxide piezoelectric layer can have a surface area in
the range of 0.0025 mm2 to 0.2 mm2. In certain embodiments, the
shapes, thicknesses and sizes of the various elements are selected to
yield a resonant frequency in the range of 0.2 GHz to 10 GHz.

[0050] The zinc oxide layer can be, for example, substantially
crystalline. In one embodiment, the zinc oxide layer is substantially
crystalline with its wurzite C axis substantially perpendicular to its
opposed surfaces.

[0051] The measurement can be performed at a wide variety of temperatures.
It may be desirable to include a temperature sensor (e.g., a thermistor)
near the film bulk acoustic wave resonator, in order to allow temperature
calibration. Accordingly, in certain embodiments, the determination of
the concentration of ethanol, the concentration of acetone, or both in
the gaseous sample takes into account the temperature of the measurement.

[0052] As described in more detail below, the zinc oxide-based film bulk
acoustic wave resonator sensor can discriminate acetone from ethanol.
Accordingly, in certain embodiments of the invention, the gaseous sample
contains both acetone and ethanol.

[0053] As described in more detail below, the zinc oxide-based film bulk
acoustic wave resonator sensor will exhibit opposite frequency shifts for
ethanol and acetone. As mentioned above, resistivity-based alcohol
sensors (such as resistive zinc oxide alcohol sensors) will provide
changes in resistance in the same direction with ethanol and acetone. One
example of a resistivity-based metal oxide sensor is described in Kim,
K., et al., Sens. Actuators. B., vol. 123, p. 318 (2007), which is hereby
incorporated herein by reference in its entirety. Similarly, Righettoni
and his coworkers developed a Si-doped WO3 nanoparticle film-based
acetone sensor with minimal response to ethanol. Anal. Chem., vol. 82,
pp. 3581-3587 (2010), which is hereby incorporated herein by reference in
its entirety. Exposing a gaseous sample to both a zinc-oxide based film
bulk acoustic wave resonator sensor and a resistivity-based sensor and
comparing the results will allow the person of skill in the art to cancel
out the effect of acetone on the measurement of ethanol and provide a
more accurate ethanol measurement; and can allow the person of skill in
the art to cancel out the effect of ethanol on the measurement of acetone
and provide a more accurate acetone measurement. Accordingly, in one
embodiment of the methods described herein, a method further includes:
providing a resistivity-based ethanol and/or acetone sensor; exposing the
resistivity-based sensor to the gaseous sample at substantially the same
time as the film bulk acoustic wave resonator is exposed to the gaseous
sample; determining the resistivity of the resistivity-based sensor; and
using the resistivity of the resistivity-based sensor along with the
resonant frequency of the film bulk acoustic wave sensor in determining
the concentration of ethanol, the concentration of acetone, or both. The
use of both the resistivity of the resistivity-based sensor and the
resonant frequency of the film bulk acoustic wave sensor in determining
the concentration of ethanol in certain embodiments can be used to
substantially cancel out any effect of acetone on the determination of
the concentration of ethanol. Similarly, the use of both the resistivity
of the resistivity-based sensor and the resonant frequency of the film
bulk acoustic wave sensor in determining the concentration of acetone in
certain embodiments can be used to substantially cancel out any effect of
ethanol on the determination of the concentration of acetone.

[0054] In another embodiment, ultraviolet light can be used to alter the
sensing performance of the film bulk acoustic wave resonator. Ultraviolet
(UV) light can degrade the response to ethanol, but enhance the response
to acetone. Accordingly, by determining resonant frequency both with and
without UV radiation, the person of skill in the art can substantially
cancel out the effect of acetone on the measurement of ethanol, and/or
substantially cancel out the effect of ethanol on the measurement of
acetone. The determination with and without UV radiation can be performed
sequentially using a single film bulk acoustic wave resonator, or using
two different, closely spaced film bulk acoustic wave resonators, one
with a UV source configured to illuminate it. See L. Peng, T. Xie, M.
Yang, P. Wang, D. Xu, S. Pang, and D. Wang, Sens. Actuators. B., 131
(2008), pp. 660-664, which is hereby incorporated herein by reference in
its entirety.

[0055] In certain embodiments, the film acoustic wave sensor is configured
to change its resonant frequency substantially linearly with ethanol
concentration throughout the range of about 100 ppm to about 250 ppm.
This range is especially relevant to detecting the breath alcohol content
of a person at 0.08 grams of alcohol/210 liters breath (which corresponds
to about 186 ppm ethanol). Accordingly, in certain embodiments, the
gaseous sample is the breath of a human subject.

[0056] The person of skill in the art can use standard calibration
techniques to determine the concentration of ethanol. A computer or
microprocessor can, for example, be used to convert the determined
resonant frequency or frequencies into an ethanol concentration (e.g., a
blood alcohol content), an acetone concentration (e.g., a blood acetone
concentration) or both. As the person of skill in the art, one or more
calibration curves can be used in performing the calculations.

[0057] In another aspect of the invention, a breath alcohol analyzer
comprises a film bulk acoustic wave resonator having a zinc oxide
piezoelectric layer; and a circuit adapted to determine an alcohol
concentration of a gaseous sample using a resonant frequency measured by
the resonant frequency measuring circuit. The film bulk acoustic wave
resonator can, for example, be as described herein. The resonant
frequency measuring circuit can be coupled to a system for determining a
concentration of ethanol in a gaseous sample (e.g., a blood alcohol
content), as described above.

[0058] In another aspect of the invention, a breath acetone analyzer
comprises a film bulk acoustic wave resonator having a zinc oxide
piezoelectric layer; and a circuit adapted to determine an acetone
concentration of a gaseous sample using a resonant frequency measured by
the resonant frequency measuring circuit. The film bulk acoustic wave
resonator can, for example, be as described herein. The resonant
frequency measuring circuit can be coupled to a system for determining a
concentration of acetone in a gaseous sample (e.g., a blood acetone
content), as described above.

Examples

[0059] Certain aspects of the invention are described in more detail in
the following Examples. The Examples demonstrate ethanol and acetone
sensing using a zinc oxide based film bulk acoustic wave resonator. The
resonant frequency of the film bulk acoustic wave resonator decreases as
the concentration of ethanol increased with a detection limit around 1
ppm. The resonant frequency of the film bulk acoustic wave resonator
increases as the concentration of acetone increased with a detection
limit around 4 ppm. Accordingly, these two gases can be distinguished due
to their opposite response. Furthermore, the fact that the sensor can
detect the presence of ethanol in the mixture of acetone and ethanol
validates its selectivity. Ultraviolet (UV) light was applied to monitor
its effects on the gas sensing performance of the film bulk acoustic wave
resonator. It degraded the response to ethanol, while enhanced the
response to acetone.

[0060] The schematic structure of the film bulk acoustic wave resonator
used in the first set of experiments described herein is shown in FIG. 4.
A sputtered ZnO film acts both as the ethanol sensitive layer and the
piezoelectric actuation layer for the film bulk acoustic wave resonator
sensor. The resonant frequency of the film bulk acoustic wave resonator
was around 1.4 GHz. The quality factor (Q) of the film bulk acoustic wave
resonator was about 550. Top and bottom schematic perspective views of
the film bulk acoustic wave resonator are provided in FIG. 5, and a
photomicrograph is shown in FIG. 6. The shape of the resonator is roughly
pentagonal with each side measuring about 75 μm. The sensing area was
about 0.026 mm2.

[0061] The film bulk acoustic wave resonator of FIGS. 4-6 is built on a
silicon nitride diaphragm (0.6 μm thick). A radio-frequency sputtered
zinc oxide (ZnO) film (1.2 μm thick) acted both as the gas sensitive
layer and the piezoelectric actuation layer for the film bulk acoustic
wave resonator. The zinc oxide film was formed as wurzite, with its C
axis alignment perpendicular to the plane of the zinc oxide layer, as
evidenced by the x-ray diffraction pattern of FIG. 7.

[0062] The top and bottom electrodes were formed from Au (0.2 μm thick)
and Al (0.2 μm thick), respectively.

[0063] The fabrication process of the film bulk acoustic wave resonator
sensor was as follows. In the first step, a silicon nitride layer was
deposited on a silicon wafer (100) with low-pressure chemical vapor
deposition (LPCVD). The silicon nitride film was patterned by reactive
ion etching (RIE). Then the Si wafer was etched from the backside
anisotropically in potassium hydroxide (KOH) to form a cavity, extending
substantially to the silicon nitride film, thereby suspending it over the
cavity. Next, the bottom Al electrode was deposited by electron-beam
evaporation and patterned on top of the silicon nitride film by wet
chemical etching. Zinc oxide was RF sputtered and etched to form the
desired pattern. The last step was the electron-beam deposition and
patterning of the top Au electrode by lift-off. Notably, the top
electrode did not form a completely conformal coating on the zinc oxide
as a result of the surface roughness of the film. Accordingly, there are
discontinuities in the top electrode that can allow gases to reach the
zinc oxide.

[0064] The device was encapsulated in a chamber to control the gas
concentration. The device was tested on a probe station with
Ground-Signal-Ground 150 micron pitch probes from Cascade Microtech Inc.
The calibration was carried out with an impedance standard substrate
using a short-open-load (SOL) method. The resonant frequency was
monitored with an Agilent E5071C network analyzer and recorded using a
LabVIEW program. The concentration of the ethanol and acetone was
calculated from the volume of the vapor and that of the chamber. We also
did calibration using Draeger gas detection pumps and tubes as reference.

[0065] The response of the film bulk acoustic wave resonator to ethanol is
shown in FIG. 8. The resonant frequency decreased as the concentration of
ethanol increased. With 120 ppm ethanol, the frequency shift was -23.2
kHz. As the concentration increased to 720 ppm, the frequency shift was
-34 kHz, reaching saturation.

[0066] The response of the film bulk acoustic wave resonator to acetone is
shown in FIG. 9. For 55 ppm acetone, the frequency upshift was 14 kHz. As
the concentration increased to 220 ppm, the frequency upshift rose to 30
kHz, reaching saturation.

[0067] In another experiment, ethanol was mixed with different amounts of
acetone to test the selectivity of the sensor. FIG. 10(a) provides the
total concentrations of acetone and ethanol; the volume ratio between
ethanol and acetone changed from 1:1 to 1:5. FIG. 10(b) provides
frequency shift data for the concentrations of FIG. 10(a). Notably the
film bulk acoustic wave resonator was able to detect the presence of
ethanol with a frequency drop for all the samples. Moreover, although the
acetone concentration increased by fivefold across the experimental data,
the frequency shift only increased by less than 1 kHz. Accordingly, the
zinc oxide based system has high selectivity for ethanol.

[0068] The effects of UV irradiation on the gas sensing performance of the
film bulk acoustic wave resonator was monitored. UV radiation degraded
the response to ethanol, as shown in FIG. 11. Without UV radiation, 120
ppm ethanol vapor caused a frequency shift of -23.2 kHz; with UV
radiation (wavelength 365 nm, 850 μW/cm2), the response was -9.6
kHz. In contrast, UV radiation enhanced the response to acetone, as shown
in FIG. 12. Without UV radiation, 120 ppm acetone vapor the frequency
shift was +6.8 kHz; with UV radiation (wavelength 365 nm, 850
μW/cm2), the response increased to +30 kHz. Moreover, the
response saturated even at 120 ppm. Without intending to be bound by
theory, the inventors propose the following mechanism for the effect of
UV radiation on the response to ethanol and acetone. UV has a
photocatalytic effect on the interaction between both acetone and ethanol
with the surface adsorbed oxygen ions to enhance the reaction. Therefore,
for acetone, the frequency upshift increased, and saturated even with a
concentration as low as 120 ppm. For ethanol, however, as the reaction
between ethanol and oxygen ions was enhanced, more water was generated
and adsorbed on the film surface. The water prevented ethanol from
diffusing into the zinc oxide film for further reaction, resulting in a
smaller response.

[0069] The schematic structure of an alternate film bulk acoustic wave
resonator for use in the present invention is shown in schematic
cross-sectional view in FIG. 13. A fabrication process flow is shown in
schematic view in FIG. 14. First, (a) low-stress silicon-rich silicon
nitride is deposited (0.3 μm) on the silicon wafer. Then, (b) the
backside silicon nitride is patterned with reactive ion etching (RIE) and
the silicon is anisotropically etched through with potassium hydroxide to
form the silicon nitride diaphragm. Next, (c) layers of chromium and gold
are deposited (0.01 μm/0.1 μm) onto the backside. On the top side
(d) the piezoelectric semiconductor material, zinc oxide (ZnO), is
sputter-deposited (0.62 μm) and patterned on the silicon nitride
diaphragm. This step is followed by (e) RIE etching of vias through the
silicon nitride, exposing the backside metal. Finally, (f) chromium and
gold are deposited (0.01 μm/0.1 μm) and patterned on the topside,
providing a topside connection to the backside metal.

[0070] Unless clearly excluded by the context, all embodiments disclosed
for one aspect of the invention can be combined with embodiments
disclosed for other aspects of the invention, in any suitable
combination.

[0071] It will be apparent to those skilled in the art that various
modifications and variations can be made to the present invention without
departing from the scope of the invention. Thus, it is intended that the
present invention cover the modifications and variations of this
invention provided they come within the scope of the appended claims and
their equivalents.